CB1 and CB2 receptors are novel molecular targets for Tamoxifen and 4OH-Tamoxifen

Tamoxifen (Tam) is classified as a selective estrogen receptor modulator (SERM) and is used for treatment of patients with ER-positive breast cancer. However, it has been shown that Tam and its cytochrome P450-generated metabolite 4-hydroxy-Tam (4OH-Tam) also exhibit cytotoxic effects in ER-negative breast cancer cells. These observations suggest that Tam and 4OH-Tam can produce cytotoxicity via estrogen receptor (ER)-independent mechanism(s) of action. The molecular targets responsible for the ER-independent effects of Tam and its derivatives are poorly understood. Interestingly, similar to Tam and 4OH-Tam, cannabinoids have also been shown to exhibit anti-proliferative and apoptotic effects in ER-negative breast cancer cells, and estrogen can regulate expression levels of cannabinoid receptors (CBRs). Therefore, this study investigated whether CBRs might serve as novel molecular targets for Tam and 4OH-Tam. We report that both compounds bind to CB1 and CB2Rs with moderate affinity (0.9-3 μM). Furthermore, Tam and 4OH-Tam exhibit inverse activity at CB1 and CB2Rs in membrane preparations, reducing basal G-protein activity. Tam and 4OH-Tam also act as CB1/CB2R-inverse agonists to regulate the downstream intracellular effector adenylyl cyclase in intact cells, producing concentration-dependent increases in intracellular cAMP. These results suggest that CBRs are molecular targets for Tam and 4OH-Tam and may contribute to the ER-independent cytotoxic effects reported for these drugs. Importantly, these findings also indicate that Tam and 4OH-Tam might be used as structural scaffolds for development of novel, efficacious, non-toxic cancer drugs acting via CB1 and/or CB2Rs.

Sp-2-propylthio-ATP-α-B and Sp-2-propylthio-ATP-α-B,β-γ-dichloromethylene are novel potent and specific agonists of the human P2Y₁₁ receptor

The human P2Y₁₁ nucleotide receptor mRNA was found in virtually all human tissues, and the receptor serves many physiological roles, such as immune response regulation. The Ala-87-Thr-P2Y₁₁ receptor single nucleotide polymorphism was linked to increased risk for acute myocardial infarction. To facilitate the development of new therapeutic applications involving cells expressing several P2 receptor subtypes, the availability of specific and potent agonists is mandatory. Here, we synthesized a series of novel adenine nucleotide derivatives, based upon the potent P2Y₁₁ receptor agonists AR-C67085. Features of the novel nucleotide derivatives are a propylthio substitution at C2-adenine and a Pα-borano or Pα-thio substitution of non-bridging oxygen atom. The latter substitutions introduce a chiral center at the α-phosphate. Sp-isomers of Pα-borano- and Rp-isomers of Pα-thio-substituted nucleotides are preferred by the P2Y₁₁ receptor. As recently reported by us, diastereoselectivity of the P2Y₁₁ receptor is opposite to that of the P2Y₁ receptor. Therefore, we exploit this characteristic to increase nucleotide selectivity. At the P2Y₁₁ receptor, the Sp-isomers of 2-propylthio-ATP-α-B (2B) and 2-propylthio-ATP-α-B,β-γ-dichloromethylene (4B) were the most potent of the novel nucleotide series, with EC₅₀ values of 0.03 μM for both, being ca. 80-fold more potent than 2-propylthio-ATP and ATP (EC₅₀ = 2.6 μM). We conclude that the borano-substitution at the α-phosphate of 2-propylthio-ATP enhances nucleotide potency at the P2Y₁₁ receptor. The combination with a Pβ-Pγ-dichloromethylene group in 4B results in a nucleotide, which shows higher selectivity for the P2Y₁₁ receptor over the P2Y₁₁ receptor than 2B making it the most promising of the novel P2Y₁₁ receptor agonists.